1,4-3,6-dianhydro-hexitols constitute a series of compounds which have been known since about 1880 and have been described in:
1. Fauconier, Bull Soc. Chem. (1884) 41, 119;
2. L. F. Wiggins, J. Chem. Soc. 1945, 4;
3. Haworth, Heath and Wiggins, J. Chem. Soc. 1944, 155;
4. R. Montgomery and L. F. Wiggins, J. Chem. Soc. 1947, 433, and
5. J. C. Goodwin, J. E. Hodge and D. Weisleder, Carbohyd. Res. 79, 133 (1980).
It is a common feature of the syntheses of these compounds that they are carried out in solvents, the intramolecular elimination of water from the hexitols being brought about by acid substances. The solvent-free syntheses also known in the literature make use of marginally volatile acids such as, e.g., sulfuric acid.
The following table gives a representative summary of various solvents and catalysts used and illustrates the efficiency of the synthesis method when used to produce 1,4-3,6-dianhydro-mannitol (the dextro isomer of which is commonly called isomannide in the literature and represented by the structure ##STR1## which is generally difficult to obtain:
______________________________________ Yield of Solvent/Catalyst Isomannide Literature ______________________________________ Dichloroglycerol .about.36% (2) Conc. Hydrochloric acid 35% (3) Conc. Hydrochloric acid 40% (3) Conc. Hydrochloric acid 25% (4) Ethylacetate/dioxane 35% (5) ______________________________________
The main disadvantages of the above processes lie in the unsatisfactory yields and correspondingly large quantities of resin-like distillation residues produced and in the long reaction times frequently required (up to 85 hours). These disadvantages eliminate the possibility of preparing, for example, isomannide or isosorbide (1,4-3,6-dianhydro-D-sorbitol of the formula ##STR2##
on a large technical scale, particularly since the raw material cost of mannitol and sorbitol required as starting compounds is considerable when compared with the usual cost of starting materials in the chemical industry.
It was, therefore, an object of the present invention to improve the preparation of dianhydrohexitols ("isohexitols" or dianhydro-hexites) from hexitols. It was intended, in particular, that the yield should be substantially higher than that obtained in known processes in order to provide the possibility of economically-acceptable production of dianhydro-hexitols on an industrial scale.
It has now surprisingly been found that in the process according to the present invention isohexitols are obtainable from hexitols in high yields by an unexpectedly selective reaction if the use of solvents is eliminated and a gaseous hydrogen halide, preferably hydrogen chloride, is reacted with crystalline or syrupy hexitol.